(Not Applicable)
The spark gap device is a traditional solution to high voltage, high current, switching applications. U.S. Pat. No. 4,475,055 of G. Boettcher shows a spark gap device including a conducting anode and cathode separated by an insulator. The device is triggered by ionizing a gas between the anode and cathode, causing a conductive plasma to be generated between the electrodes.
The typical spark-gap device utilizes a pair of electrodes in a vacuum spaced from each other by an insulator. When a high voltage is placed across the electrodes, the electrically-stressed insulator may undergo surface-flashover at an applied field more than an order of magnitude below the bulk dielectric strength of the insulator.
J. Brainard et al, Electron avalanche and surface charging on alumina insulators during pulsed high-voltage stress, Journal of Applied Physics, Vol. 45, No. 8, August 1974, pp. 3260-3264, modeled insulator surface charging on high-voltage diodes. The object of this understanding was to prevent such discharges.
R. Anderson et al, Mechanism of pulsed surface flashover involving electron-stimulated desorption, Journal of Applied Physics, Vol. 51, No. 3, 1980, pp. 1414-1421, showed that surface flashover of dielectrics results from a gas that is desorbed from the insulator by impinging electrons resulting from a breakdown in vacuum.
The spark gap switch relies on electron transport through a gas. A surface breakdown switch is shown in U.S. Pat. No. 5,821,705 of G. Caporaso et al which provides faster switching by surface breakdown of the device.
There is not universal agreement as to what happens at each stage of a surface flashover. R. Anderson, Review of Surface Flashover Theory, Sandia National Laboratories report SAND89-1276C, July 1989 (available from NTIS, Springfield, Va. 22161), reviews some of the theories that had been proposed to explain each of cathode-initiated and anode-initiated surface flashover. The content of this report is incorporated herein by reference. Some additional theories are referenced below.
A. Neuber et al, Dielectric Surface Flashover in Vacuum at 100KV, IEEE Transactions on Dielectrics and Insulation, Vol. 6, No. 4, Aug. 1999, pp. 512-515, indicates that surface flashover in vacuum at room temperature xe2x80x9cis usually started by field emission of electrons from the cathode, phase (1). A subsequent electron avalanche development is governed by secondary electron emission from the dielectric surface, followed by electron induced outgassing of adsorbed gas molecules, phase (2). A gas breakdown was found to form in the expanding gas layer above the surface, leading to the final discharge, phase (3).xe2x80x9d
G. Masten et al, Plasma Development in the Early Phase of Vacuum Surface Flashover, IEEE Transactions on Plasma Science, Vol. 22, No. 6, Dec. 1994, pp. 1034-1042, used laser deflection from a test setup in vacuum and concluded that deflection measurements xe2x80x9cimply that charge-carrier amplification within the developing discharge occurs above the surface of the insulator, in a region of neutral particles desorbed or otherwise ejected from the insulator surface.xe2x80x9d
T. Engle et al, Surface-Discharge Switch Design: The Critical Factor, IEEE Transactions on Electron Devices, Vol. 38, No. 4, April 1991, pp. 740-744, notes that xe2x80x9cmost designers [of high power, low impedance closing switches] prefer and use other types of switches (e.g. spark gaps, thyratrons, ignitrons, etc.). This is because the surface-discharge-switch (SDS) suffers from poor voltage holdoff recovery (caused by decomposition of the switching dielectric) and from dielectric xe2x80x98punch-throughxe2x80x99 (caused by dielectric erosion). Thus the selection of the switching dielectric is the critical factor which must be considered by the designer if the SDS is to have a long and trouble-free lifetime.xe2x80x9d
Historically, the surface flashover (or discharge) switch has not been too successful because designs that favor the required electron avalanching usually have poor voltage hold-off capability. In other words, the desired switching voltage is approximately the same as the voltage that is across the switch while it is open circuited. A desired switch should have switching voltage significantly lower than the hold-off voltage, to prevent unintended discharge of the switch.
R. Koss et al, Partial Discharge in a High Voltage Experimental Test Assembly, Sandia National Laboratories report SAND98-1654, Sandia National Laboratories, July 1998, describes an observation by the inventors of an undesirable breakdown in a high voltage test assembly with an insulator between two electrodes that was designed not to break down. Protrusions on the insulator near the anode end of the device caused high fields that released electrons from the insulator that strike the anode with sufficient energy to vaporize and ionize the metal. The anode discharge is then sustained by secondary electrons from the insulator that propagate by field enhancement to the cathode, opposite the normal direction of discharge. A related breakdown is discussed in the aforementioned Review of Surface Flashover Theory. 
It is an object of this invention to provide a high voltage, high current, surface flashover switch utilizing an insulator that is designed not to avalanche at the cathode.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, a high voltage switch in accordance with the invention may include an electrically conductive cathode having inner and outer spaced surfaces; and an electrically conductive anode having inner and outer spaced surfaces, the inner surface of the anode facing the inner surface of the cathode. A hollow tubular insulator is sealed at one end to the inner surface of the anode and at an opposite end to the inner surface of the cathode, defining a volume which is evacuated. A controllable generator of electrons adjacent the cathode causes the switch to change from a non-conducting to a conducting state as a result of an anode initiated breakdown.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.